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Wednesday, July 20, 2011

Converting a Clothes Dryer to Use Solar Heated Attic Air

Randy has done a really interesting new project that reduces the energy used to dry clothes at his house by a factor of five!

His scheme converts a regular dryer to use hot air supplied by his unique attic solar heating collector instead of heating the air within the dryer. A duct from his attic collector supplies air to the revamped intake of the converted dryer. The dryer exhaust air is vented out through the wall in the normal way.

The converted dryer on the left and the new intake ducting on the right.
The upper duct connects to the attic solar collector.

Dryers are major users of energy in homes. A Canada study shows that dryers typically use 930 KWH a year to do 416 dryer loads (2.23 KWH per load). Nearly all of this heat energy is simply expelled outdoors (wasted). In addition, as the dryer vents air outside, it pulls fresh air into the house which has to be heated or cooled (depending on season) to room temperature -- this can easily add another 300 KWH plus per year. Since refrigerators have become much more efficient over the years, dryers now stand out as the applicance using the most energy in many homes. It is amazing to me that Energy Star has taken no action in this area -- but, I guess that's another story.

Our dryer loads take about 1 hour on a regular home electric dryer, so I guess there is a bit more tumble time. I suppose it depends on the temperature you select?But, I can't remember ever losing clothes to tumber wear? Seems like worn knees, stains,... are what do my clothes in.

I guess that if the longer time turns out to be an issue, one could use the solar as the base heating, and then boost the temperature up with the dryers heating element?

Hi Phil,Just as a quick guess on your question about using air heating solar panels to feed the dryer:A dryer uses about 2.2 KWH of energy to dry a load. Some of this just runs the motor, but most is for heating the air -- lets say 2 KWH for heating. If you figure an hour of drying time, then this would be an average heating requirement of 2000 watts.

Full sun is about 1000 watts per sq meter, and solar collectors are in the area of 50% efficient, so, the heat output under good collection conditions is about 500 watts/sqm. If the solar drying is done on the same 1 hour time table, it would take about (2000 w/500 w/sm) = 4 sq meters, or 43 sqft. If you were willing to settle for a longer drying time, then the collector could be proportionately smaller.It may be that a good solar design for a dryer does not look like a conventional dryer at all -- I think in Europe its fairly common to use clothes drying closets in which the clothes hang with a stream of warm air flowing over them -- this might make a better solar dryer?

The same collector could be used for space heating while not being used for drying.Either way, you are also saving the energy that is needed to reheat the cold air that the dryer pulls into the house as it vents outside.You could even experiment with venting the dryer inside if you live in a low humidity climate. This would make double use of the solar heated collector -- drying clothes and then space heating.

Consumer Reports has found that dryers with a moisture sensor tend to recognize when laundry is dry more quickly than machines that use a traditional thermostat. Because they don't subject clothing to unnecessary heat, moisture-sensor models are easier on fabrics.